The Czochralski method (hereafter, referred to as the “CZ method”) is a method in which a single-crystal ingot is grown by pulling it from raw material melt such as silicon melt placed in a crucible. In order to grow a single crystal with high controllability, the surface level of the raw material melt (hereafter, referred to as the “melt level”) must be measured accurately to adjust the level position according to the growth of the single crystal.
In a silicon single crystal pulling device using the CZ method, in particular, a heat shield is usually provided for controlling heat radiation from a heater and silicon melt and for straightening gas introduced into a CZ furnace.
The thermal history and impurity concentration (e.g. oxygen concentration) of single crystals to be pulled can be made constant by controlling the relative position (i.e. distance) between the lower surface of the heat shield and the melt level.
The prior art has proposed several melt level measurement methods in this regard.
The under-mentioned Patent Document 1 discloses a method based on the principle of triangulation, in which accurate melt level measurement is realized by using a melt surface shape constantly generated by motion in the melt surface and cause the same to function as a kind of reflector. Hereafter, this measurement method is referred to as a direct reflection method.
FIGS. 7A and 7B are a diagram for explaining a laser beam trajectory according to the direct reflection method. FIG. 7A is a schematic diagram showing the laser beam trajectory as viewed from a side (in an X-Y plane). FIG. 7B is a schematic diagram showing the laser beam trajectory as viewed from the front (in an X-Z plane). It should be understood that although a laser beam is guided by a rotating mirror 9 and a prism 11 in FIG. 7A, the rotating mirror 9 and the prism 11 are omitted in FIG. 7B since the laser beam trajectory in a Y-axis direction is perpendicular to the paper surface of FIG. 7B. Other parts and components that are not essential in terms of the triangulation are also omitted for simplification of illustration.
In FIGS. 7A and 7B, a silicon material 3 is molten within a crucible 2 provided in a CZ furnace 1, and a silicon single crystal 4 is being grown while being pulled up and rotated. A heat shield 5 is arranged around the outside of the silicon single crystal 4. An interval between the inner circumference of the lower end of the heat shield 5 and the peripheral wall of the silicon single crystal 4 is represented by D, and an interval between the lower surface 6 of the heat shield 5 and the melt surface 7 is represented by L.
The invention described above employs a distance measurement unit 8 based on the principle of triangulation for measuring the level of the melt surface 7.
As shown in FIG. 7B, there are provided, in the inside of the distance measurement unit 8, a laser beam source 12 for emitting a laser beam and a photo detector 13 for receiving return light that has been reflected. There are provided, in the photo detector 13, a lens 13a for collecting an incident laser beam and a one-dimensional CCD sensor 13b for detecting the collected laser beam.
A laser beam emitted from the distance measurement unit 8 is reflected by the rotating mirror 9, passes through an entrance window 10, and is applied to the melt surface 7 via a prism 11 provided in the CZ furnace 1.
The rotating mirror 9 is rotated clockwise or anticlockwise (indicated by the arrow S1 in the figure) to scan the position on the melt surface 7 where the laser beam is applied in a radial direction of the crucible 3 (indicated by the arrow S2 in the figure), so that the return light reflected by the melt surface 7 is received by the photo detector at a predetermined frequency via the prism 11, the entrance window 10, and the rotating mirror 9. According to the direct reflection method, as described above, a laser beam emitted by the laser beam source is directly applied to the melt surface 7, and the return light reflected by the melt surface 7 is directly received by the photo detector 13.
When the melt level of the melt surface 7 is at a position A1, the laser beam reflected by the melt surface 7 is detected at measurement coordinates X1 of the one-dimensional CCD sensor 13. This means that the measurement coordinates X1 of the one-dimensional CCD sensor 13 correspond to the melt level A1. Likewise, when the melt level is at a position A2, the laser beam reflected by the melt surface 7 is detected at measurement coordinates X2 of the one-dimensional CCD sensor 13. This means that the measurement coordinates X2 of the one-dimensional CCD sensor 13 correspond to the melt level A2. In this manner, according to the principle of triangulation, the melt level can be calculated from the measurement coordinates detected by the one-dimensional CCD sensor 13.
Although the angle of incidence and the angle of reflection of a laser beam at the melt surface 7 (both represented by an angle θ1) are shown greater than actual one in the figure, the angle θ1 actually is a small angle of a few degrees. This applies to other cases as well.
The direct reflection method, which uses a melt surface shape generated in the melt surface as a reflector, is suitable for a case in which a single crystal is pulled without applying a magnetic field to the periphery of the melt. The method also enables measurement regardless of the magnitude of interval L. Since the return light is reflected light directly from the melt surface 7, high laser power is not required.
The under-mentioned Patent Document 2 discloses a method for measuring a melt level by scattering a laser beam emitted by a laser beam source once at the lower surface of a heat shield, and reflecting the laser beam twice at the melt surface. This measurement method is hereafter referred to as the return reflection method. In the return reflection method, a magnetic field must be applied to the melt to eliminate undulation of the melt surface, that is, to smoothen the melt surface for the purpose of efficient utilization of the two reflections and one scattering.
FIGS. 8A and 8B are a diagram for explaining a position measuring method based on the return reflection method.
FIG. 8A is a schematic diagram showing a laser beam trajectory as viewed from a side (in the X-Y plane). FIG. 8B is a schematic diagram showing the laser beam trajectory as viewed from the front (in the X-Z plane). Although a laser beam is guided by a rotating mirror 9 and a prism 11 in FIG. 8A, the rotating mirror 9 and the prism 11 are omitted in FIG. 8B since the laser beam trajectory in a Y-axis direction is perpendicular to the paper surface of FIG. 8B. Other parts and components that are not essential in terms of the triangulation are also omitted for simplification of illustration.
As shown in FIG. 8B, there are provided, in the inside of a distance measurement unit 8, a laser beam source 12 for emitting a laser beam and a photo detector 13 for receiving return light that has been reflected. There are arranged, in the photo detector 13, a lens 13a for collecting an incident laser beam and a one-dimensional CCD sensor 13b for detecting the collected laser beam.
As shown in FIGS. 8A and 8B, a laser beam emitted by the laser beam source 12 is reflected by the rotating mirror 9 and the prism 11 and applied to a melt surface 7. The laser beam thus applied is reflected by the melt surface 7 (melt level A4), and the reflected light is applied to the lower surface 6 of a heat shield 5 provided above the melt surface 7. The applied laser beam is scatted at a scattering point 6a on the lower surface 6 of the heat shield, and the scattered light is again applied to the melt surface 7. The laser beam thus applied is again reflected by the melt surface 7, and the reflected light is finally received by the photo detector 13.
This means that the laser beam received by the photo detector 13 is reflected light of a laser beam that has been applied to the melt surface 7 from the scattering point 6a of the lower surface 6 of the heat shield, and hence it is detected by the photo detector 13 as a laser beam emitted from a scattering point 3a on an apparent reflection surface.
The angle of incidence and the angle of reflection of a laser beam always take a same value in the X-Z plane in FIG. 8B. Based on simple geometrical consideration, the scattering point 3a on the apparent reflection surface and the scattering point 6a on the lower surface 6 of the heat shield 5 are in a positional relationship in which they are symmetrical with respect to the melt level A4 (mirror relationship). The apparent reflection surface is hereafter referred to as the “melt level A3”. In FIG. 8B, the apparent heat shield 5b that is symmetrical to the heat shield 5 with respect to the actual melt level A4 is also shown by a dot line in order to help understanding of an apparent laser beam trajectory. Accordingly, the interval between the lower surface 6 of the heat shield 5 and the apparent melt level A3 is 2L.
The position of the lower surface 6 of the heat shield 5 can be obtained, for example, by measuring the position of the upper surface 9 of the lower end portion of the heat shield 5. In FIG. 8B, the position of the laser beam reflection point 9a on the upper surface 9 of the lower end portion of the heat shield 5 corresponds to the measurement coordinates X9 of the one-dimensional sensor. If it is assumed that the distance M between the upper surface 9 and the lower surface 6 has been measured in advance, the position of the lower surface 6 of the heat shield can be obtained based on the measurement coordinates X9 and the distance M.
The interval L can be obtained as a half of the relative distance 2L between the position of the lower surface 6 of the heat shield and the apparent melt level A3. The actual melt level A4 can be obtained as a value obtained by adding the interval L to the apparent melt level A3.
The return reflection method, utilizing scattered light scattered by the heat shield, has an advantage that the photo detector is able to receive light at a high light receiving probability.
Patent Document 1: Japanese Patent Application Laid-Open No. 2000-264779
Patent Document 2: WO01/083859